Gas sensor

ABSTRACT

A gas sensor including a sensor element constituted by an oxygen-ion conductive solid electrolyte as a main component and detecting a predetermined gas component in a measurement gas includes: an external communication part having an opening opened to the outside, and introducing the measurement gas from the outside under a predetermined diffusion resistance; an internal space communicating with the external communication part; a first electrode formed on a surface of the internal space; a second electrode formed in a space different from the internal space; and a pumping cell operable to pump out oxygen existing in the internal space when a predetermined voltage is applied between the first electrode and the second electrode. The thickness of the external communication part is 50% or more and 100% or less of the thickness of the internal space.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas sensor including a sensor elementand measuring a predetermined gas component in a measurement gas.

2. Description of the Background Art

Conventionally, various measuring apparatuses have been used forrecognizing a concentration of a desired gas component in a measurementgas. For example, as a device for measuring a NOx concentration in ameasurement gas such as a combustion gas, known is a gas sensor havingan electrochemical pumping cell structured by forming a Pt electrode anda Rh electrode on an oxygen-ion conductive solid electrolyte layer, suchas a zirconia (ZrO₂) layer. In this gas sensor, a diffusion control partcommunicating with the outside is formed so as to take a measurement gasfrom the outside into a sensor element and to apply a diffusionresistance suitable for measurement of a concentration of a desired gascomponent to the measurement gas.

For example, known is a gas sensor in which an opening communicatingwith the outside is provided in an end portion of a sensor element and adiffusion control part configured as a horizontally elongated slit isprovided (for example, see Japanese Patent No. 3701124). Japanese PatentNo. 3701124 discloses a gas sensor further including a gas inlet spacebefore the slit-shaped diffusion control part mentioned above. Alsoknown is a gas sensor including a diffusion control part formed by aporous body being filled in a portion of a sensor element forintroducing a measurement gas from the outside so that a diffusionresistance suitable for measurement of a concentration of a desired gascomponent can be applied to the measurement gas during take-in of themeasurement gas into the sensor element from the outside (for example,see Japanese Patent No. 3176890).

The gas sensors disclosed in Japanese Patent No. 3701124 and JapanesePatent No. 3176890 are, for example, installed in an exhaust pipe of aninternal combustion engine such as an automobile engine, and used tomeasure a concentration of a desired gas component contained in anexhaust gas. In such an internal combustion engine, in a case wheredriving thereof is stopped, water vapor contained in the exhaust gas maybe condensed to form a water droplet. Such a water droplet is not onlyaccumulated in the exhaust pipe but also adheres to the gas sensor.

Generally, a gas sensor is mounted in an exhaust pipe so as to besubstantially perpendicularly to a flow of an exhaust gas while aprotective covering having a gas inlet hole through which a gas canfreely pass is provided at the outer side of a sensor element.Therefore, in a case where a water droplet adheres to the inside of theprotective covering, a part of the water droplet is discharged throughthe gas inlet hole. However, the water droplet still stays at a bottomportion of the protective covering. Since the bottom portion of theprotective covering is in close contact with an end portion of thesensor element, when an internal combustion engine is re-driven, aconsiderable part of the water droplet staying at the bottom portion ofthe protective covering adheres to the end portion of the sensorelement. Additionally, a water droplet staying within the exhaust pipemay splash within the protective covering to adhere to the end portionof the sensor element.

As a result, the adhering of the water droplet partially cools only theend portion, to cause a temperature gradient between the end portion andthe inside of the sensor element. Moreover, a thermal stress resultingfrom this temperature gradient may cause cracking.

Additionally, water adhering to the end portion of the sensor elementmay reach an internal space through a slit-shaped diffusion control partformed in the end portion of the sensor element due to the capillarity.In a case where the internal combustion engine is driven in this state,the water rapidly vaporizes in the internal space to cause cracking inthe sensor element.

In this manner, occurrence of cracking in the sensor element largelychanges a diffusion resistance to be applied to the measurement gas,which is not preferable because a measurement accuracy is deteriorated.

SUMMARY OF THE INVENTION

The present invention is directed to a gas sensor for measuring apredetermined gas component in a measurement gas, and particularlyrelates to a structure of a sensor element forming the gas sensor.

According to the present invention, a gas sensor detecting apredetermined gas component in a measurement gas includes a sensorelement constituted by an oxygen-ion conductive solid electrolyte as amain component. The said sensor element includes: an externalcommunication part having an opening opened to the outside, andintroducing the measurement gas from the outside under a predetermineddiffusion resistance; an internal space communicating with the externalcommunication part; a first electrode formed on a surface of theinternal space; a second electrode formed in a space different from theinternal space; and a pumping cell operable to pump out oxygen existingin the internal space when a predetermined voltage is applied betweenthe first electrode and the second electrode. The thickness of theexternal communication part is 50% or more and 100% or less of thethickness of the internal space.

This can achieve a gas sensor whose measurement accuracy is stablymaintained even if water adheres to an end portion of the sensorelement.

Preferably, the sensor element included in the gas sensor according tothe present invention further includes a buffer space provided betweenthe external communication part and the internal space.

This can achieve a gas sensor in which a concentration fluctuation inthe measurement gas can be suitably suppressed even if the measurementgas is abruptly taken into the sensor element along with a pressurefluctuation in the measurement gas existing in the outside.

Preferably, in the gas sensor according to the present invention, theexternal communication part has the opening formed in a side portion oran end portion of the sensor element.

This can achieve a gas sensor which can suppress entry of water adheringto the end portion of the sensor element into the sensor element due tothe capillarity.

Therefore, an object of the present invention is to provide a gas sensorwhose measurement accuracy can be stably maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an outline of an exemplaryconfiguration of a sensor element of a gas sensor according to a firstpreferred embodiment;

FIGS. 2A, 2B, and 2C are schematic cross-sectional views showing outlinecross-sections of the sensor element of the gas sensor according to thefirst preferred embodiment;

FIG. 3 is a schematic cross-sectional view showing an exemplaryconfiguration of the gas sensor according to the first preferredembodiment;

FIGS. 4A, 4B, and 4C are schematic cross-sectional views showing outlinecross-sections of a sensor element of a gas sensor according to a secondpreferred embodiment;

FIG. 5 is a perspective view showing an outline of an exemplaryconfiguration of a sensor element of a gas sensor according to a thirdpreferred embodiment;

FIGS. 6A, 6B, and 6C are schematic cross-sectional views showing outlinecross-sections of the sensor element of the gas sensor according to thethird preferred embodiment;

FIG. 7 is a perspective view showing an outline of an exemplaryconfiguration of a sensor element of a gas sensor according to a fourthpreferred embodiment;

FIGS. 8A, 8B, and 8C are schematic cross-sectional views showing outlinecross-sections of the sensor element of the gas sensor according to thefourth preferred embodiment;

FIGS. 9A, 9B, and 9C are schematic cross-sectional views showing amodification of the gas sensor according to the present invention;

FIG. 10 is a diagram showing the relationship between the fracturestrength and the structure of an external communication part;

FIG. 11 is a diagram showing the relationship between the fracturestrength and the width of an external communication part of a gas sensoraccording to an example equivalent to the first preferred embodiment;

FIG. 12 is a diagram showing the relationship between the fracturestrength and the thickness of an external communication part of a gassensor according to an example equivalent to the second preferredembodiment;

FIG. 13 is a diagram showing the relationship between the fracturestrength and the thickness of an external communication part of a gassensor according to an example equivalent to the third preferredembodiment;

FIG. 14 is a diagram showing the relationship between the fracturestrength and the width of an external communication part of a gas sensoraccording to an example equivalent to the fourth preferred embodiment;and

FIG. 15 is a diagram showing a result of measurement of a response timefor gas sensors according to examples and a comparative example.

DETAILED DESCRIPTION OF THE INVENTION First Preferred Embodiment

<Outline Configuration of Gas Sensor>

Firstly, an outline configuration of a gas sensor 100 will be described.

FIG. 1 is an external appearance perspective view showing an outline ofan exemplary configuration of a sensor element 101 of the gas sensor100. FIG. 1 shows a right-hand xyz coordinate system in which thelongitudinal direction of the sensor element 101 is defined as thex-axis (hereinafter, the same is true). FIGS. 2A, 2B, and 2C areschematic cross-sectional views showing outline cross-sections of thesensor element 101 as sectioned at different positions shown in FIG. 1,for explaining the structure of a gas distribution part 111. In FIGS.2A, 2B, and 2C, for simplification, components other than the gasdistribution part 111 are omitted. FIG. 2A shows a cross-section assectioned along the arrow A-A′ of FIG. 1. FIG. 2B shows a cross-sectionas sectioned along the arrow B-B′ of FIG. 1. FIG. 2C shows across-section as sectioned along the arrow C-C′ of FIG. 1. FIG. 3 is aschematic cross-sectional view showing an example of a detailedconfiguration of the gas sensor 100.

The sensor element 101 is an elongated plate-shaped element having astructure in which six layers, namely, a first substrate layer 1, asecond substrate layer 2, a third substrate layer 3, a first solidelectrolyte layer 4, a spacer layer 5, and a second solid electrolytelayer 6, are laminated in the mentioned order from the bottom side seenin FIG. 3, each of the layers being formed as an oxygen-ion conductivesolid electrolyte layer such as a zirconia (ZrO₂) layer. The solidelectrolyte forming these six layers is densely airtight. The sensorelement 101 is manufactured by, for example, performing a predeterminedprocess and printing a circuit pattern on ceramic green sheets, each ofwhich corresponds to each of the layers, then laminating the greensheets, then cut the laminated body into element units, and furthermorebaking the laminated body to integrate it.

Between a lower surface of the second solid electrolyte layer 6 and anupper surface of the first solid electrolyte layer 4 at one end portionof the sensor element 101, an external communication part 11, a firstinternal space 20, a first diffusion control part 30, and a secondinternal space 40 are adjacently formed in the mentioned order so as tobe in communication with one another.

The external communication part 11 is a space within the sensor element101 provided by hollowing out the spacer layer 5, in which its upperportion is defined by the lower surface of the second solid electrolytelayer 6 or an internal surface 5 a of the spacer layer 5, its lowerportion is defined by the upper surface of the first solid electrolytelayer 4 or an internal surface 5 b of the spacer layer 5, and its sideportion is defined by side surfaces 5 c and 5 d of the spacer layer 5.In more detail, the position where the external communication part 11 isformed is adjusted in accordance with the degree of a diffusionresistance suitable for measurement of a nitrogen oxide (NOx)concentration, which will be described later.

The first internal space 20 and the second internal space 40 are alsospaces within the sensor element 101 provided by hollowing out thespacer layer 5, in which their upper portions are defined by the lowersurface of the second solid electrolyte layer 6, their lower portionsare defined by the upper surface of the first solid electrolyte layer 4,and their side portions are defined by the side surfaces of the spacerlayer 5.

The first diffusion control part 30 is configured as two horizontallylong slits (whose openings are elongated in a y-axis direction). A partextending from the external communication part 11 to the second internalspace 40 is also referred to as a gas distribution part 111.

In the other end portion of the sensor element 101, at a position whichis farther from an end portion 101 a of the element than the gasdistribution part 111 is, a reference gas inlet space 43 is providedbetween an upper surface of the third substrate layer 3 and a lowersurface of the spacer layer 5. A side portion of the reference gas inletspace 43 is defined by a side surface of the first solid electrolytelayer 4. As a reference gas for measuring a NOx concentration, forexample, air is introduced into the reference gas inlet space 43.

An air introduction layer 48 is constituted by porous alumina. Thereference gas is introduced through the reference gas inlet space 43into the air introduction layer 48. The air introduction layer 48 isfowled so as to cover a reference electrode 42.

The reference electrode 42 is an electrode formed so as to be interposedbetween the upper surface of the third substrate layer 3 and the firstsolid electrolyte layer 4. As described above, the air introductionlayer 48 leading to the reference gas inlet space 43 is provided aroundthe reference electrode 42. By using the reference electrode 42, anoxygen concentration (oxygen partial pressure) in the internal space canbe measured, as will be described later.

<Details of Gas Distribution Part and Associated Parts>

The external communication part 11 applies a predetermined diffusionresistance suitable for measurement of a nitrogen oxide (NOx)concentration to the measurement gas introduced through an opening 11 aformed in the end portion 101 a of the sensor element from the outside,and then guides the resistant measurement gas to the first internalspace 20 communicating with the external communication part 11 through acommunication part 11 b.

A width a of the external communication part 11 in the y-axis directionand a thickness b thereof in a z-axis direction are defined inaccordance with the degree of the diffusion resistance to be applied tothe measurement gas introduced from the opening 11 a into the firstinternal space 20.

Here, it is preferable that the thickness b of the externalcommunication part 11 is 50% to 100% of a thickness (hollow-chamberthickness) d of the first internal space 20 in the z-axis direction, andthe width a of the external communication part 11 is 5% to 60%(desirably 10% to 40%) of a width (hollow-chamber width) c of the firstinternal space 20 in the y-axis direction.

The thickness b less than 50% of the thickness d is not preferable,because the opening 11 a has a reduced opening area so that wateradhering to the end portion 101 a of the sensor element may easily enterthe first internal space 20 due to the capillarity. The width a lessthan 5% of the width c is not preferable, because the opening 11 a has areduced opening area so that water adhering to the end portion 101 a ofthe sensor element may easily enter the first internal space 20 due tothe capillarity. The width a more than 60% of the width c makes itdifficult that a diffusion resistance suitable for measurement of thenitrogen oxide (NOx) concentration is applied to the measurement gas.

In this manner, in the gas sensor 100, the width a and the thickness bof the external communication part 11 are defined within the rangementioned above, thereby suppressing entry of water adhering to the endportion 101 a of the sensor element into the first internal space 20.

The first internal space 20 is provided as a space for adjusting theoxygen partial pressure in the measurement gas introduced through theexternal communication part 11. This oxygen partial pressure is adjustedby an operation of a main pumping cell 21.

The main pumping cell 21 is an electrochemical pumping cell constitutedby an inside pump electrode 22, an outside pump electrode 23, and a partof the second solid electrolyte layer 6 interposed between theseelectrodes. The inside pump electrode 22 has a ceiling electrode portion22 a provided on a substantially entire part of the lower surface of thesecond solid electrolyte layer 6 facing the first internal space 20. Theoutside pump electrode 23 is provided in a region on an upper surface ofthe second solid electrolyte layer 6 corresponding to the ceilingelectrode portion 22 a, so as to be exposed to the outside.

The inside pump electrode 22 is formed over the upper and lower solidelectrolyte layers (the second solid electrolyte layer 6 and the firstsolid electrolyte layer 4) which define the first internal space 20, andthe spacer layer 5 which provides a side wall to the first internalspace 20. To be specific, the ceiling electrode portion 22 a is formedon the lower surface of the second solid electrolyte layer 6 whichprovides a ceiling surface to the first internal space 20. A bottomelectrode portion 22 b is formed on the upper surface of the first solidelectrolyte layer 4 which provides a bottom surface to the firstinternal space 20. A side electrode portion (not shown) connecting theceiling electrode portion 22 a to the bottom electrode portion 22 b isformed on side wall surfaces (inner surfaces) of the spacer layer 5which form both side wall portions of the first internal space 20. Thus,the inside pump electrode 22 has a tunnel-like shape at a location wherethe side electrode portion is disposed.

Each of the inside pump electrode 22 and the outside pump electrode 23is formed as a porous cermet electrode (for example, a cermet electrodeincluding Pt containing Au by 1% and zirconia). The inside pumpelectrode 22 which is brought into contact with the measurement gas isformed using a material having a weakened reduction ability with respectto a NOx component in the measurement gas, or having no reductionability with respect to the NOx component in the measurement gas.

In the main pumping cell 21, a desired pump voltage Vp0 is appliedbetween the inside pump electrode 22 and the outside pump electrode 23to make a pump current Ip0 flow in a positive direction or a negativedirection between the inside pump electrode 22 and the outside pumpelectrode 23, and this allows oxygen existing within the first internalspace 20 to be pumped out to the outside or oxygen existing in theoutside to be pumped into the first internal space 20.

In order to detect an oxygen concentration (oxygen partial pressure) inthe atmosphere of the first internal space 20, an electrochemical sensorcell, in other words, a main-pump-controlling oxygen-partial-pressuredetection sensor cell 80 is formed with the inside pump electrode 22,the second solid electrolyte 6, the spacer layer 5, the first solidelectrolyte 4, the third substrate layer 3, and the reference electrode42.

The oxygen concentration (oxygen partial pressure) in the first internalspace 20 can be recognized by measuring an electromotive force V0 of themain-pump-controlling oxygen-partial-pressure detection sensor cell 80.Moreover, the pump current Ip0 is controlled by feedback-controlling Vp0so as to maintain the electromotive force V0 constant. Thereby, theoxygen concentration in the first internal space 20 can be maintained ata predetermined constant value.

The first diffusion control part 30 applies a predetermined diffusionresistance to the measurement gas whose oxygen concentration (oxygenpartial pressure) has been controlled in the first internal space 20 bythe operation of the main pumping cell 21, and guides the measurementgas to the second internal space 40.

The second internal space 40 is provided as a space for performing aprocess concerning measurement of the nitrogen oxide (NOx) concentrationin the measurement gas introduced through the first diffusion controlpart 30. In the second internal space 40 in which the oxygenconcentration in the measurement gas is adjusted by an auxiliary pumpingcell 50, the NOx concentration is measured by the operation of ameasuring pumping cell 41.

In the second internal space 40, the auxiliary pumping cell 50 performsfurther adjustment of oxygen partial pressure on the measurement gaswhose oxygen concentration (oxygen partial pressure) has been adjustedin advance in the first internal space 20 and which has then beenintroduced through the first diffusion control part 30. This enables anoxygen concentration in the second internal space 40 to be accuratelymaintained constant. Therefore, the gas sensor 100 can measure a NOxconcentration with a high accuracy.

The auxiliary pumping cell 50 is an auxiliary electrochemical pumpingcell constituted by an auxiliary pump electrode 51, the outside pumpelectrode 23 (not limited to the outside pump electrode 23 but may beany appropriate electrode positioned outside the sensor element 101),and the second solid electrolyte layer 6. The auxiliary pump electrode51 has a ceiling electrode portion 51 a provided on a substantiallyentire part of the lower surface of the second solid electrolyte layer 6facing the second internal space 40.

Similarly to the inside pump electrode 22 provided in the first internalspace 20, the auxiliary pump electrode 51 has a tunnel-like shape andprovided in the second internal space 40. That is, the ceiling electrodeportion 51 a is formed on the second solid electrolyte layer 6 whichprovides a ceiling surface to the second internal space 40. A bottomelectrode portion 51 b is formed on the first solid electrolyte layer 4which provides a bottom surface to the second internal space 40. A sideelectrode portion (not shown) connecting the ceiling electrode portion51 a to the bottom electrode portion 51 b is formed on both wallsurfaces of the spacer layer 5 which provides side walls to the secondinternal space 40.

Similarly to the inside pump electrode 22, the auxiliary pump electrode51 is unwed using a material having a weakened reduction ability withrespect to a NOx component in the measurement gas, or having noreduction ability with respect to the NOx component in the measurementgas.

In the auxiliary pumping cell 50, a desired voltage Vp1 is appliedbetween the auxiliary pump electrode 51 and the outside pump electrode23, and this allows oxygen existing in the atmosphere of the secondinternal space 40 to be pumped out to the outside or oxygen existing inthe outside to be pumped into the second internal space 40.

In order to control oxygen partial pressure in the atmosphere of thesecond internal space 40, an electrochemical sensor cell, in otherwords, an auxiliary-pump-controlling oxygen-partial-pressure detectionsensor cell 81 is formed with the auxiliary pump electrode 51, thereference electrode 42, the second solid electrolyte layer 6, the spacerlayer 5, the first solid electrolyte layer 4, and the third substratelayer 3.

A variable power source 52 causes the auxiliary pumping cell 50 toperform pumping. The variable power source 52 is voltage-controlledbased on an electromotive force V1 which is detected by theauxiliary-pump-controlling oxygen-partial-pressure detection sensor cell81. Therefore, the oxygen partial pressure in the atmosphere of thesecond internal space 40 is lowered to have substantially no influenceon the NOx measurement.

At the same time, a pump current Ip1 of the auxiliary pumping cell 50 isused for a control of the electromotive force of themain-pump-controlling oxygen-partial-pressure detection sensor cell 80.Specifically, the pump current Ip1 is inputted as a control signal tothe main-pump-controlling oxygen-partial-pressure detection sensor cell80, and its electromotive force V0 is controlled, so that a gradient ofthe oxygen partial pressure in the measurement gas introduced into thesecond internal space 40 through the first diffusion control part 30 ismaintained so as to be always constant. When used as a NOx sensor, theoxygen concentration in the second internal space 40 is maintained at aconstant value of approximately 0.001 ppm, by the operations of the mainpumping cell 21 and the auxiliary pumping cell 50.

Thus, in the gas sensor 100, by operating the main pumping cell 21 andthe auxiliary pumping cell 50, the oxygen partial pressure is alwaysmaintained to be a constant low value (value having substantially noinfluence on the measurement of NOx).

The measurement gas whose oxygen concentration has been adjusted in thesecond internal space 40 is measured for the NOx concentration by theoperation of the measuring pumping cell 41. The measuring pumping cell41 is an electrochemical pumping cell constituted by a measuringelectrode 44, the outside pump electrode 23, the second solidelectrolyte layer 6, the spacer layer 5, and the first solid electrolytelayer 4. The measuring electrode 44 is provided at a position on theupper surface of the first solid electrolyte layer 4 facing the secondinternal space 40.

The measuring electrode 44 is a porous cermet electrode. The measuringelectrode 44 also functions as a NOx reducing catalyst which reduces NOxexisting in the atmosphere of the second internal space 40. Themeasuring electrode 44 is covered with a second diffusion control part45.

The second diffusion control part 45 is a film constituted by a porousbody containing alumina (Al₂O₃) as a main component. The seconddiffusion control part 45 serves to limit the amount of NOx flowing intothe measuring electrode 44, and also functions as a protective film ofthe measuring electrode 44.

The measuring pumping cell 41 can pump out oxygen generated bydecomposition of nitrogen oxide in the atmosphere around the measuringelectrode 44, and detects the amount of the generated oxygen as a pumpcurrent Ip2.

In order to detect oxygen partial pressure around the measuringelectrode 44, an electrochemical sensor cell, in other words, ameasuring-pump-controlling oxygen-partial-pressure detection sensor cell82 is formed with the second solid electrolyte layer 6, the spacer layer5, the first solid electrolyte layer 4, the third substrate layer 3, themeasuring electrode 44, and the reference electrode 42. A variable powersource 46 is controlled based on an electromotive force V2 detected bythe measuring-pump-controlling oxygen-partial-pressure detection sensorcell 82.

As described above, the measurement gas introduced into the secondinternal space 40 reaches the measuring electrode 44 through the seconddiffusion control part 45 while the oxygen partial pressure in themeasurement gas is being controlled. Nitrogen oxide in the measurementgas existing around the measuring electrode 44 is reduced (2NO→N₂+O₂),to generate oxygen. The generated oxygen is pumped by the measuringpumping cell 41. At this time, a voltage Vp2 of the variable powersource is controlled such that a control voltage V2 detected by themeasuring-pump-controlling oxygen-partial-pressure detection sensor cell82 can be maintained constant. The amount of oxygen generated around themeasuring electrode 44 is proportional to a nitrogen-oxide concentrationin the measurement gas. Thus, the nitrogen-oxide concentration in themeasurement gas is calculated by using the pump current Ip2 of themeasuring pumping cell 41.

In other words, in the gas sensor 100, the NOx concentration in themeasurement gas can be recognized based on the pump current Ip2 whichflows when the oxygen generated by the reduction of NOx is pumped out bythe measuring pumping cell 41 substantially in proportion to the NOxconcentration in the measurement gas.

An electrochemical sensor cell 83 is formed with the second solidelectrolyte layer 6, the spacer layer 5, the first solid electrolytelayer 4, the third substrate layer 3, the outside pump electrode 23, andthe reference electrode 42. By an electromotive force Vref obtained bythe sensor cell 83, oxygen partial pressure in the measurement gasexisting in the outside of the sensor can be detected.

<Heater Part>

Furthermore, in order to enhance an oxygen-ion conductivity of the solidelectrolyte, the sensor element 101 includes a heater part 70 servingfor a temperature control for heating and keeping warm the sensorelement 101. The heater part 70 includes a heater electrode 71, a heater72, a through hole 73, a heater insulating layer 74, and a pressurediffusion hole 75

The heater electrode 71 is an electrode formed in contact with a lowersurface of the first substrate layer 1. By connecting the heaterelectrode 71 to an external power source, electrical power can besupplied to the heater part 70 from the outside.

The heater 72 is an electric resistor interposed vertically between thesecond substrate layer 2 and the third substrate layer 3. The heater 72is connected to the heater electrode 71 via the through hole 73. Theheater 72 generates heat when power is supplied from the outside throughthe heater electrode 71, and heats and keeps warm the solid electrolytewhich forms the sensor element 101.

The heater 72 is buried over the entire area extending from the firstinternal space 20 to the second internal space 40, so that thetemperature of the entire sensor element 101 can be adjusted at atemperature at which the solid electrolyte is activated.

The heater insulating layer 74 is an insulating layer constituted by aninsulator such as alumina and formed on upper and lower surfaces of theheater 72. The heater insulating layer 74 is formed for the purpose ofproviding an electrical insulation between the second substrate layer 2and the heater 72 and an electrical insulation between the thirdsubstrate layer 3 and the heater 72.

The pressure diffusion hole 75 is formed through the third substratelayer 3, and communicates with the reference gas inlet space 43. Thepressure diffusion hole 75 is formed for the purpose of relieving a risein the internal pressure which is involved in a temperature rise in theheater insulating layer 74.

As described above, in the gas sensor 100 according to this preferredembodiment, the size of the external communication part 11 having theopening 11 a formed in the end portion 101 a of the sensor element 101is suitably set, thereby suppressing entry of water adhering to the endportion 101 a of the sensor element into the internal space due to thecapillarity. This can consequently suppress occurrence of cracking inthe sensor element 101 which may otherwise be caused by rapidvaporization of the water in the internal space.

Therefore, in the gas sensor 100 according to this preferred embodiment,a deterioration in the measurement accuracy which may be caused byoccurrence of cracking in the sensor element 101 can be suitablysuppressed. In other words, even if water adheres to the end portion 101a of the sensor element 101 during the operation of the gas sensor 100,the measurement accuracy is stably maintained.

In a case where the thickness b of the external communication part 11 ismade equal to the thickness d of the first internal space 20, the sensorelement 101 can be formed by, in the above-mentioned green-sheetprocess, forming an opening serving as the external communication part11 simultaneously with forming openings serving as the first internalspace 20 and the second internal space 40 in the ceramic green sheetserving as the spacer layer 5 by punching, and then performing the sameprocess steps as described above.

In a case where the thickness b of the external communication part 11 ismade smaller than the thickness d of the first internal space 20, agreen sheet which will be laminated between the first solid electrolytelayer 4 and the spacer layer 5 or between the spacer layer 5 and thesecond solid electrolyte layer 6 may be separately prepared in additionto the green sheets serving as the six layers, then a cutout may beformed in the ceramic green sheet by punching in the above-mentionedgreen-sheet process, then the green sheets including this green sheetmay be laminated, and then the same process steps as described above maybe performed.

Alternatively, in the above-mentioned green-sheet process, a paste withsublimation properties to form the external communication part 11 may beprinted on the first solid electrolyte layer 4 or the spacer layer 5,when printing a predetermined circuit pattern on the ceramic greensheets serving as the respective layers. In this case, the paste issublimed at a time of baking, to form the external communication part11.

As described above, according to this preferred embodiment, the externalcommunication part 11 having a predetermined size is provided in the endportion 101 a of the sensor element 101, to thereby achieve a gas sensorwhose measurement accuracy is stably maintained even if water adheres tothe end portion 101 a of the sensor element 101 during the operation.

Second Preferred Embodiment

In a second preferred embodiment, a configuration in which a gasdistribution part is different from that of the gas sensor 100 accordingto the first preferred embodiment will be described.

An external appearance perspective view showing an exemplaryconfiguration of a sensor element 201 of a gas sensor 200 according tothe second preferred embodiment is identical to that of the gas sensor100 according to the first preferred embodiment. Therefore, here, FIG. 1is referred to.

FIGS. 4A, 4B, and 4C are schematic cross-sectional views showing outlinecross-sections of the sensor element 201 sectioned at differentpositions shown in FIG. 1, for explaining the structure of a gasdistribution part 112. In FIGS. 4A, 4B, and 4C, for simplification,components other than the gas distribution part 112 are omitted. FIG. 4Ashows a cross-section as sectioned along the arrow A-A′ of FIG. 1. FIG.4B shows a cross-section as sectioned along the arrow B-B′ of FIG. 1.FIG. 4C shows a cross-section as sectioned along the arrow C-C′ ofFIG. 1. The same components as those of the gas sensor 100 according tothe first preferred embodiment are denoted by the same correspondingreference numerals, and descriptions and drawings thereof are omitted.

In the sensor element 201, a buffer space 91 and a third diffusioncontrol part 92 are provided between the external communication part 11and the first internal space 20 of the sensor element 101 according tothe first preferred embodiment. In this preferred embodiment, a partextending from the external communication part 11 to the second internalspace 40 is also referred to as a gas distribution part 112.

Similarly to the external communication part 11, the buffer space 91 isa space within the sensor element 101 provided by hollowing out thespacer layer 5, in which its upper portion is defined by the lowersurface of the second solid electrolyte layer 6, its lower portion isdefined by the upper surface of the first solid electrolyte layer 4, andits side portions are defined by the side surface of the spacer layer 5.Here, the buffer space 91 is formed such that its width w in the y-axisdirection can be larger than the width a of the external communicationpart 11.

Providing the buffer space 91 can reduce an influence of a pressurefluctuation of the measurement gas existing in the outside (a pulsationof exhaust gas pressure, in a case where the measurement gas is anautomobile exhaust gas) while the measurement gas is being introducedfrom the outside of the sensor element 201 into the first internal space20 of the sensor element 201. Thus, even if the measurement gas isabruptly taken due to the pressure fluctuation, a concentrationfluctuation in the measurement gas introduced into the first internalspace 20 is substantially negligible.

The third diffusion control part 92 applies a predetermined diffusionresistance to the measurement gas introduced from the buffer space 91 tothe first internal space 20. The third diffusion control part 92 isconfigured as two horizontally long slits (whose openings are elongatedin the y-axis direction).

In the sensor element 201, the external communication part 11, thebuffer space 91, and the third diffusion control part 92, as a whole,apply a predetermined diffusion resistance suitable for measurement ofthe nitrogen oxide (NOx) concentration to the measurement gas introducedfrom the opening 11 a to the second internal space 20.

The values of the width a and the thickness b which define the value ofthe diffusion resistance applied in the external communication part 11may be set in consideration of the relationship with the diffusionresistance to be applied in the third diffusion control part 92. In thispreferred embodiment, similarly to the first preferred embodiment, it ispreferable that the thickness b of the external communication part 11 is50% to 100% of the thickness d of the first internal space 20 in thez-axis direction, and the width a of the external communication part 11is 5% to 60% (desirably 10% to 40%) of the width c of the first internalspace 20 in the y-axis direction. This can suppress occurrence of thecapillarity.

As described above, according to this preferred embodiment, similarly tothe first preferred embodiment, even if water adheres to the end portion101 a of the sensor element 101 during the operation, the measurementaccuracy is stably maintained. Moreover, since the buffer space 91 andthe third diffusion control part 92 are provided, a gas sensor can beachieved in which a concentration fluctuation in the measurement gas issuppressed even if the measurement gas is abruptly taken into the sensorelement 201 along with a pressure fluctuation of the measurement gasexisting in the outside.

Third Preferred Embodiment

In a third preferred embodiment, the position where an externalcommunication part is provided is different from that of the gas sensor100 according to the first preferred embodiment.

FIG. 5 is an external appearance perspective view showing an outline ofan exemplary configuration of a sensor element 301 of a gas sensor 300according to the third preferred embodiment. FIGS. 6A, B, and 6C areschematic cross-sectional views showing outline cross-sections of thesensor element 301 as sectioned at different positions shown in FIG. 5,for explaining the structure of a gas distribution part 113. In FIGS.6A, B, and 6C, for simplification, components other than the gasdistribution part 113 are omitted. FIG. 6A shows a cross-section assectioned along the arrow A-A′ of FIG. 5. FIG. 6B shows a cross-sectionas sectioned along the arrow B-B′ of FIG. 5. FIG. 6C shows across-section as sectioned along the arrow C-C′ of FIG. 5. The samecomponents as those of the gas sensor 100 according to the firstpreferred embodiment and the gas sensor 200 according to the secondpreferred embodiment are denoted by the same corresponding referencenumerals, and descriptions and drawings thereof are omitted.

In the sensor element 301, external communication parts 12 are providedin side portions 301 b of the sensor element 301, instead of theexternal communication part 11 provided in the end portion 101 a of thesensor element 101 according to the first preferred embodiment. In thispreferred embodiment, a part extending from the external communicationparts 12 to the second internal space 40 is also referred to as a gasdistribution part 113.

Similarly to the external communication part 11, the externalcommunication parts 12 are spaces within the sensor element 301 providedby hollowing out the spacer layer 5, in which their upper portions isdefined by the lower surface of the second solid electrolyte layer 6 orthe internal surface 5 a of the spacer layer 5, their lower portions isdefined by the upper surface of the first solid electrolyte layer 4 orthe internal surface 5 b of the spacer layer 5, and their side portionsare defined by side surfaces 5 e and 5 f of the spacer layer 5. In moredetail, the positions where the external communication parts 12 areformed are adjusted in accordance with the degree of the diffusionresistance suitable for measurement of the nitrogen oxide (NOx)concentration.

A width e of the external communication part 12 in the x-axis directionand a thickness f of the external communication part 12 in the z-axisdirection are defined in accordance with the degree of the diffusionresistance to be applied to the measurement gas introduced from anopening 12 a into the first internal space 20.

Here, in the external communication part 12, similarly to the externalcommunication part 11, it is preferable that the thickness f is 50% to100% of the thickness d of the first internal space 20 in the z-axisdirection and the width e is 5% to 60% (desirably 10% to 40%) of thewidth c of the first internal space 20 in the y-axis direction.

In this manner, in the gas sensor 300, the width e and the thickness fof the external communication part 12 are defined within the rangementioned above, thereby suppressing entry of water adhering to the sideportion 301 b of the sensor element 301 into the first internal space20.

In the external communication part 12, it is preferable that a distancet1 in the x-axis direction from an end of the first internal space 20 atthe element end portion side to the communication parts 12 b is equal toor less than 20% of a length t2 of the first internal space 20 in thex-axis direction. The distance t1 exceeding 20% makes it difficult toadjust oxygen partial pressure in the measurement gas within the firstinternal space 20. The value of t1 may be 0, because it allows theoxygen partial pressure to be adjusted more effectively.

As described above, in the gas sensor 300 according to this preferredembodiment, the external communication parts 12 having the openings 12 aformed in the side portions 301 b of the sensor element 301 areprovided, to thereby suppress entry of water adhering to an end portion301 a of the sensor element 301 due to the capillarity. Moreover,suitably setting the size of the external communication part 12 alsosuppresses entry of water adhering to the side portion 301 b of thesensor element 301 due to the capillarity. This can consequentlysuppress occurrence of cracking in the sensor element 301 which mayotherwise be caused by rapid vaporization of the water in the internalspace.

Additionally, in the gas sensor 300 according to this preferredembodiment, the external communication part 12 having the opening 12 aformed in the side portion 301 b of the sensor element 301 is providedinstead of an external communication part having an opening formed inthe end portion 301 a of the sensor element 301. This improves thestrength of a portion near the end portion 301 a of the sensor element301. As a result, occurrence of cracking in the sensor element 301 issuppressed which may otherwise be caused by a thermal stress wateradhering to the end portion 301 a of the sensor element 301.

Therefore, in the gas sensor 300 according to this preferred embodiment,a deterioration in the measurement accuracy which may be caused byoccurrence of cracking in the sensor element 301 can be suitablysuppressed. In other words, even if water adheres to the end portion 301a of the sensor element 301 during the operation of the gas sensor 300,the measurement accuracy is stably maintained.

As described above, according to this preferred embodiment, the externalcommunication part 12 having a predetermined size is provided in theside portion 301 b of the sensor element 301, to thereby achieve a gassensor whose measurement accuracy can be stably maintained even if wateradheres to the end portion 301 a of the sensor element 301 during theoperation.

Fourth Preferred Embodiment

In a fourth preferred embodiment, a configuration of a gas distributionpart is different from that of the gas sensor 300 according to the thirdpreferred embodiment.

FIG. 7 is an external appearance perspective view showing an outline ofan exemplary configuration of a sensor element 401 of a gas sensor 400according to the fourth preferred embodiment. FIGS. 8A, 8B, and 8C areschematic cross-sectional views showing outline cross-sections of thesensor element 401 as sectioned at different positions shown in FIG. 7,for explaining the structure of a gas distribution part 114. In FIGS.8A, 8B, and 8C, for simplification, components other than the gasdistribution part 114 are omitted. FIG. 8A shows a cross-section assectioned along the arrow A-A′ of FIG. 7. FIG. 8B shows a cross-sectionas sectioned along the arrow B-B′ of FIG. 7. FIG. 8C shows across-section as sectioned along the arrow C-C′ of FIG. 7. The samecomponents as those of the gas sensor 100 according to the firstpreferred embodiment or the gas sensor 300 according to the thirdpreferred embodiment are denoted by the same corresponding referencenumerals, and descriptions and drawings thereof are omitted.

In the sensor element 401, a buffer space 91 and a third diffusioncontrol part 92 are provided between the external communication parts 12and the first internal space 20 of the sensor element 301 according tothe third preferred embodiment. In this preferred embodiment, a partextending from the external communication parts 12 to the secondinternal space 40 is also referred to as the gas distribution part 114.In this preferred embodiment, the buffer space 91 is provided such thatits width t4 in the x-axis direction can be larger than the width e ofthe external communication part 12. As a result, a buffering effectsimilar to that of the second preferred embodiment can be obtained.

In the sensor element 401, the external communication parts 12, thebuffer space 91, and the third diffusion control part 92, as a whole,apply a predetermined diffusion resistance suitable for measurement ofthe nitrogen oxide (NOx) concentration to the measurement gas introducedfrom the opening 12 a to the second internal space 20.

The values of the width e and the thickness f which define the value ofthe diffusion resistance applied in the external communication parts 12may be set in consideration of the relationship with the diffusionresistance to be applied in the third diffusion control part 92. In thispreferred embodiment, similarly to the third preferred embodiment, it ispreferable that the thickness f of the external communication parts 12is 50% to 100% of the thickness d of the first internal space 20 in thez-axis direction, and the width e of the external communication parts 12is 5% to 60% (desirably 10% to 40%) of the width c of the first internalspace 20 in the y-axis direction. This can suppress occurrence of thecapillarity.

In the external communication parts 12, it is preferable that a distancet3 in the x-axis direction from an end of the buffer space 91 at theelement end portion side to the communication part 12 b is equal to orless than 50% of a length t4 of the buffer space 91 in the x-axisdirection. The distance t3 exceeding 50% makes it difficult to cancel arapid change of the concentration fluctuation in the measurement gaswithin the buffer space 91.

As described above, according to this preferred embodiment, similarly tothe third preferred embodiment, a gas sensor can be achieved in whichthe measurement accuracy can be stably maintained even if water adheresto the end portion 401 a of the sensor element 401 during the operation,and moreover, since the buffer space 91 and the third diffusion controlpart 92 are provided, a concentration fluctuation in the measurement gascan be suitably suppressed even if the measurement gas is abruptly takeninto the sensor element 401 along with a pressure fluctuation in themeasurement gas.

<Modification>

Although in the above description, the first diffusion control part 30and the third diffusion control part 92 are formed as horizontallyelongated slits, applications of the present invention are not limitedthereto. The first diffusion control part 30 and the third diffusioncontrol part 92 may be formed into other shapes.

Additionally, although in the third preferred embodiment and the fourthpreferred embodiment, one external communication part is provided ineach of the side portions of the sensor element, applications of thepresent invention are not limited thereto. Two or more externalcommunication parts may be provided in each of the side portions of thesensor element. Alternatively, the external communication part may beprovided in only one of the side portions.

In a case where the thickness of the external communication part issmaller than the thickness of the first internal space, the externalcommunication part may be formed in either upper or lower portion of thespacer layer 5, as shown in FIG. 9A and FIG. 9B, or may be formedsubstantially in the middle of the spacer layer 5, as shown in FIG. 9C.

EXAMPLES Example 1

A water drop test was performed on a gas sensor A which is an example ofthe gas sensor 100 according to the first preferred embodiment, a gassensor B which is an example of the gas sensor 200 according to thesecond preferred embodiment, gas sensors C and D which are examples ofthe gas sensor 300 according to the third preferred embodiment, gassensors E and F which are examples of the gas sensor 400 according tothe fourth preferred embodiment, and a gas sensor G which is acomparative example.

In the gas sensor C, the distance t1 is 0. In the gas sensor D, thedistance t1 is 10% of the length t2. In the gas sensor E, the distancet3 is 0. In the gas sensor F, the distance t3 is 10% of the length t4.The gas sensor G is a conventional gas sensor in which slit-shapedexternal communication part having an opening is provided in an endportion of a sensor element. In the gas sensor G, the width of theexternal communication part is equal to the hollow-chamber width, andthe thickness of the external communication part is 5% of thehollow-chamber thickness.

In the water drop test, in a state where the gas sensor was driven whilethe sensor element was heated at a predetermined temperature, a waterdroplet was dropped to or near an end portion of the sensor element, andwhether cracking occurred in the sensor element or not was examined. Inthe test, the amount of dropped water droplet was increased untilcracking occurred in the sensor element. At a time point when crackingoccurred in the sensor element, the driving of the gas sensor wasstopped and the amount of water droplet at this time point was measured.A cracking water droplet amount ratio is a value represented by a ratioof a water droplet amount causing cracking in each of the gas sensors Ato F with respect to a water droplet amount causing cracking in the gassensor G.

FIG. 10 shows a result of measurement of a cracking water droplet amountin each of the gas sensors A to F. In the gas sensor A, the width a is20% of the width c, and the thickness b is 100% of the thickness d. Inthe gas sensor B, the width a is 10% of the width c, and the thickness bis 100% of the thickness d. In the gas sensor C, the width e is 20% ofthe width c, and the thickness f is 100% of the thickness d. In the gassensor D, the width e is 30% of the width c, and the thickness f is 50%of the thickness d. In the gas sensor E, the width e is 20% of the widthc, and the thickness f is 100% of the thickness d. In the gas sensor F,the width e is 15% of the width c, and the thickness f is 80% of thethickness d. For each of the gas sensors A to G, the number of measuredgas sensors was five.

In FIG. 10, the circles indicate average values of measurement values,and the lines above and below the circles indicate the maximum valuesand the minimum values of the measurement values. As shown in FIG. 10,it is observed that the cracking water droplet amounts of the gassensors A to F were larger than that of the gas sensor G. This resultmeans that the gas sensors A to F have fracture strengths larger thanthat of the gas sensor G.

Example 2

In this example, for examining the relationship between the size of theexternal communication part and the fracture strength, a plurality ofgas sensors were prepared for each of the preferred embodiments, theplurality of gas sensors being different from one another in the ratioof the size of the external communication part to the size of the hollowchamber. Then, the same water drop test as that of the example 1 wasperformed.

FIG. 11 shows a result of measurement of the cracking water dropletamount of each of the eight types of gas sensors corresponding to thefirst preferred embodiment. The eight types of gas sensors are differentfrom one another in the width a of the external communication part 11.FIG. 12 shows a result of measurement of the cracking water dropletamount of each of the five types of gas sensors corresponding to thesecond preferred embodiment. The five types of gas sensors are differentfrom one another in the thickness b of the external communication part12. FIG. 13 shows a result of measurement of the cracking water dropletamount of each of five types of gas sensors corresponding to the thirdpreferred embodiment. The five types of gas sensors are different fromone another in the thickness for the external communication part 12.FIG. 14 shows a result of measurement of the cracking water dropletamount of each of eight types of gas sensors corresponding to the fourthpreferred embodiment. The eight types of gas sensors are different fromone another in the width e of the external communication part 12. Foreach type of the gas sensor, the number of measured gas sensors wasfive. Here, it is to be noted that, the cases where the ratio of thewidth of the external communication part to the hollow-chamber width was100% in FIGS. 11 and 14, and the cases where the ratio of the thicknessof the external communication part to the hollow-chamber thickness was5% in FIGS. 12 and 13 were results of measurement with respect to thegas sensor G which is a comparative example.

The circles in FIGS. 11 to 14 indicate average values of measurementvalues, and the lines above and below the circles indicate the maximumvalues and the minimum values of the measurement values. As shown inFIGS. 11 to 14, it is observed that the cracking water droplet amount ofany of the gas sensors according to this example was larger than that ofthe gas sensor G according to the comparative example, irrespective ofthe width or the thickness of the external communication part. Thisresult means that any of the gas sensors according to this example had alarger fracture strength than that of the gas sensor G according to thecomparative example, irrespective of the width or the thickness of theexternal communication part. Particularly, in a case where the ratio ofthe width of the external communication part to the hollow-chamber widthwas 5% or more and 60% or less, and a case where the ratio of thethickness of the external communication part to the hollow-chamberthickness was 50% or more and 100% or less, the cracking water dropletamount was twice or more of that of the gas sensor according to thecomparative example. This indicates that providing the externalcommunication part so as to satisfy these sizes can achieve a gas sensorhaving a doubled fracture strength as compared with the conventional gassensor.

From the results of the examples 1 and 2, it is confirmed that providingthe external communication part whose size is set so as to satisfy theabove-mentioned requirements is effective in improving the fracturestrength. In both of the examples 1 and 2, it is confirmed thatproviding the external communication part having an opening in a sideportion of the sensor element (corresponding to the third and fourthpreferred embodiments) is more effective in improving the fracturestrength. This result means that providing the above-described externalcommunication part is effective in suppressing a deterioration in themeasurement accuracy which may otherwise be caused by occurrence ofcracking in the sensor element.

Example 3

In this example, the respective gas sensors of the example 1 wereexamined for the responsiveness. In a test of the responsiveness, in acase where an air-fuel ratio (a oxygen concentration in a measurementgas) of a measurement gas was changed from λ=0.9 to λ=1.1, a sensoroutput (Ip2) obtained at a time when the inside of the sensor elementwas substantially fully replaced with the measurement gas having anair-fuel ratio of λ=0.9 was defined as 0%, and the sensor output (Ip2)obtained at a time when the inside of the sensor element wassubstantially fully replaced with a measurement gas having an air-fuelratio of λ=1.1 was defined as 100%. Under this condition, a time(response time) from the time point when the sensor output (Ip2)corresponding to 33% was detected to the time point when the sensoroutput (Ip2) corresponding to 66% was detected was measured three timesfor each of the gas sensors.

FIG. 15 shows a result of measurement of the response time of each ofthe gas sensors A to G. In each of the gas sensors A to F, therelationship between the width of the external communication part andthe width c, and the relationship between the thickness of the externalcommunication part and the thickness d were the same as those of the gassensors used in the water drop test shown in FIG. 10.

In FIG. 15, the circles indicate average values of the measurementvalues, and the lines above and below the circles indicate the maximumvalues and the minimum values of the measurement values. As shown inFIG. 15, no significant difference was observed between the responsetimes of the gas sensors A to F and the resonse time of the gas sensorG.

The above-described results reveal that responsiveness equivalent to theresponsiveness of the conventional gas sensor can be obtained also bythe gas sensors A to F provided with the external communication partthat satisfies the above-mentioned requirements.

What is claimed is:
 1. A gas sensor for detecting a predetermined gascomponent in a measurement gas, said gas sensor comprising: a sensorelement constituted by an oxygen-ion conductive solid electrolyte as amain component and having an upper portion, a lower portion, opposingend portions and opposing side portions; an external communication parthaving an opening opened to the outside, and introducing the measurementgas from the outside under a predetermined diffusion resistance; aninternal space communicating with said external communication part; afirst electrode formed on a surface of said internal space; a secondelectrode formed in a space different from said internal space; apumping cell operable to pump out oxygen existing in said internal spacewhen a predetermined voltage is applied between said first electrode andsaid second electrode; and a buffer space provided between said externalcommunication part and said internal space, wherein said externalcommunication part is a space having a constant width along alongitudinal direction of the sensor element, wherein the thickness ofsaid external communication part is 50% or more and 100% or less of thethickness of said internal space, and the width of said externalcommunication part is 5% or more and 60% or less of the width of saidinternal space, wherein a width of the buffer space is larger than awidth of said external communication part, and wherein the buffer spaceis directly connected to said external communication part.
 2. The gassensor according to claim 1, wherein said external communication parthas said opening formed in at least one side portion of the opposed sideportions of said sensor element.
 3. The gas sensor according to claim 1,wherein said external communication part has said opening formed in oneend portion of the opposing end portions of said sensor element.
 4. Thegas sensor according to claim 1, wherein said internal space furthercomprises a first internal space and a second internal space, said firstinternal space being in communication with said external communicationpart and said second internal space; and said pumping cell furthercomprises a main pumping cell and an auxiliary pumping cell, whereinsaid first internal space is in communication with said externalcommunication part under a predetermined diffusion resistance, saidsecond internal space is in communication with said first internal spaceunder a predetermined diffusion resistance, said main pumping cell hassaid first electrode provided in said first internal space, and saidauxiliary pumping cell has a third electrode provided in said secondinternal space.
 5. The gas sensor according to claim 4, wherein saidsensor element further comprises: a fourth electrode formed on a surfaceof said second internal space; a measuring cell including said fourthelectrode and said second electrode; and a porous diffusion layer formedon said fourth electrode and applying a predetermined diffusionresistance to said measurement gas, said main pumping cell is operableto pump out oxygen existing in said first internal space when apredetermined voltage is applied between said first electrode providedin said first internal space and said second electrode, said auxiliarypumping cell is operable to pump out oxygen existing in said secondinternal space when a predetermined voltage is applied between saidthird electrode provided in said second internal space and said secondelectrode, said fourth electrode reduces an oxide gas component in saidpredetermined gas component to which said predetermined diffusionresistance has been applied by said porous diffusion layer, saidmeasuring cell is operable to measure an electromotive force betweensaid fourth electrode and said second electrode to determine a nitrogenoxide concentration in the measurement gas using a pump current betweensaid fourth electrode and said second electrode.